Wire-drawing dies that employ diamond as the bearing material have been available for many years. Typically, a portion of a wire-drawing die that forms a die cavity for shaping the wire is made from natural diamond. Natural diamond has commonly been used for the portion of the wire-drawing die that forms the die cavity. However, the size of the die cavity is limited by the availability of suitably-sized natural diamond pieces. Moreover, the cost of natural diamond can be prohibitively high for large die cavities.
Polycrystalline diamond has also been used for the die cavity. FIG. 1 shows a side cross-sectional view of a prior art design for a wire-drawing die 10 that utilizes polycrystalline diamond. The wire-drawing die 10 includes a polycrystalline diamond compact (“PDC”) 11 formed of an inner, annular polycrystalline diamond (“PCD”) region 12 that is often bonded to an outer cobalt-cemented tungsten carbide cylinder 14. The PCD region 12 also includes a die cavity 16 configured for receiving and shaping a wire being drawn. The wire-drawing die 10 is typically encased in stainless steel to allow for handling and marking. In use, a wire 18 of a diameter d, is drawn through die cavity 16 along a wire drawing axis 20 to reduce the diameter of the wire 18 to a reduced diameter d2.
A variety of conventional processes may be used for fabricating the wire-drawing die 10. Typically, a hollow of the tungsten carbide cylinder 14 is packed with diamond particles. The tungsten carbide cylinder 14 and the diamond particles are subjected to an ultra-high-pressure, ultra-high-temperature (“HPHT”) process that melts a portion of the cobalt from tungsten carbide cylinder 14. The melted cobalt is swept into the interstitial regions of the diamond particles from a region of the tungsten carbide cylinder 14 adjacent to the diamond particles. The melted cobalt acts as a catalyst that promotes bonding of the diamond particles to form a coherent mass of polycrystalline diamond shown as the PCD region 12 in FIG. 1. Laser drilling may be used to form a hole through the sintered polycrystalline diamond that is subsequently smoothly finished to form the die cavity 16.
Increasing the size of the die cavity 16 to allow for drawing large-diameter wire and wire bundles (e.g., over about 0.5 inch diameter) tends to result in a lower quality PCD region 12. Inefficiency of cobalt diffusion by the conventional radial sweep-through technique is believed to be one factor causing this decrease in quality for the PCD region 12. Radial diffusion may fail to provide a sufficient cobalt concentration near the center of the PCD region 12, resulting in poorly bonded diamond particles in the PCD region 12 and porosity in the PCD region 12. These defects may render the PCD region 12 unusable or may decrease the lifetime of the wire-drawing die 10. Additionally, larger, more expensive diamond presses are used to sinter larger PCD regions 12. Fabrication of large-diameter superhard dies of high quality can be therefore difficult and expensive. Manufacturers and users of wire-drawing dies continue to seek improved superhard wire-drawing dies for drawing large-diameter wires and wire bundles.